ABSTRACT The aim of the present study was to analyse the composition and spatiotemporal distribution of the benthic macrofauna in the estuary of the Perizes River (state of Maranhão, Brazil). Biological and environmental samples were collected at five sampling sites along the estuary during six campaigns covering the months of September, October and November 2016 (dry period) and February, March and April 2017 (wet period). Sampling was performed with a van Veen grab sampler (682cm2). Environmental variables as temperature, pH, salinity, dissolved oxygen, saturation and water transparency were measured and related to the biological data at each sampling site. After collection the samples were sieved through a 0.5 mm mesh and the organisms fixed in a 10% formaline solution. A total of 35,597 individuals belonging to 84 taxa were identified, corresponding to Insecta, Mollusca, Crustacea, Annelida, Nematoda, Nemertea, Sipuncula and Collembola. Polychaeta showed the greatest species richness, while Tanaidacea were the most abundant. No gradient was found throughout the sampling sites with regard to the environmental variables, but marked differences were found in the temporal distribution. The temporal distribution of the benthic macrofauna was influenced by salinity and rainfall. Oligochaeta and Tanaidacea were abundant at all sampling sites. Acari were abundant in the inner portion of the estuary, while the number of Ostracoda was very high in the outer portion. The structure of the macrobenthic community differed in composition from the majority of the studies on estuaries, with the dominance of Tanaidacea and Oligochaeta. The higher abundance of Tanaidacea is related to environments rich in fine sediments with organic matter and the occurrence of this characteristics was determinant for the distribution of other taxa. The results indicate that the wet period favours the establishment of the benthic community, as evidenced by the increase in diversity and equitability.

ABSTRACT São Marcos Bay is an estuarine complex with semidiurnal tides that can reach more than 7 m during equinoctial spring tides. It is situated in the second largest Brazilian coastline and is subjected to continuous human activities holding an important port complex of Latin America. In order to contribute to the knowledge about the structure of the phytoplankton in a macrotidal systems, this study aims to evaluate the phytoplankton community and its relationship with environmental conditions in the São Marcos Bay. Five surveys in 2010 and 2011 were carried out on four sampling points during the rainy and dry seasons. Samples were taken during the flood and ebb phases at neap tides. Hydrological parameters were correlated with biological data (phytoplankton composition, abundance and biomass) using statistical analysis. The phytoplankton biomass (chlorophyll a) characterized the bay as quite productive (10.43±7.62 mg m-3) with the nanophytoplankton as the dominant fraction. Seasonal variation was observed in phytoplankton abundance with higher values (34,262±18,422 cells L-1) in the rainy season. Diatoms were the most important phytoplankton group, pointing out Nitzschia sp., Diploneis weissflogii and Synedra sp. as the dominant species. This study revealed that the composition of phytoplankton community was mainly influenced by the local dynamics, governed by macrotides, and precipitation regime that mostly contributed to the seasonal fluctuations of the environmental conditions, such as salinity, dissolved oxygen, and nutrients.